Electrode patterning for a differential PZT activator

ABSTRACT

A process for fabricating piezoelectric elements each having a wrap-around electrode to be used in a differential actuator design where electrical connection is made to the bottom electrode of the element from the top surface of the element. The wrap-around electrode is formed during the creation of the elements instead of on an element by element basis.

This application is a 371 of International Application No.PCT/US00/10601, filed Apr. 20, 2000, which claims the benefit ofProvisional Application Ser. No. 60/254,966, filed Apr. 20, 1999 for“Electrode Patterning for a Differential PZT Activator,” by John StuartWright and Zine Eddine Boutaghou.

BACKGROUND OF THE INVENTION

The present invention relates to a differential piezoelectric actuatorto be used in a disc drive, and more particularly, to the fabrication ofa differential piezoelectric actuator.

Radial track density in disc drives continues to increase, resulting inan increased need for extremely precise head positioning systems. Voicecoil motor (VCM) actuators are well-suited to effect coarse positioning,but lack the resolution to finely position and center a transducing headover a selected track. This inadequacy has led to a variety of proposalsfor a second stage microactuator to effect fine positioning in hightrack density disc drives.

The microactuator proposals have taken several forms, from anelectrostatic microactuator attached to the slider carrying thetransducing head, to a piezoelectric microactuator installed at the headsuspension mounting block at a distal end of the actuator arm.

With respect to piezoelectric microactuators, there are currently twoschemes for driving piezoelectric materials in secondary microactuators.The first is a single ended driving scheme where the piezoelectricmaterial is attached with a conductive epoxy-solder paste to a stainlesssteel suspension which acts as the bottom electrode and electricalconnection is made only to the top electrode. The second is adifferential design driving scheme where the piezoelectric material iselectrically isolated from the suspension and electrical connection ismade to both the top and bottom electrodes of the piezoelectric element.

An advantage of the single ended design is its ease of manufacturing, asonly one electrical connection to the small and fragile piezoelectricelement is necessary. However, to maintain a class II UL listing, thevoltage supplied to the piezoelectric element must be limited to +/−20volts. Such a voltage is below that necessary to achieve the desiredstroke. On the other hand, the differential design allows +/−40 volts tobe supplied to the piezoelectric element which allows the desired stroketo be achieved. The differential design, however, complicatesfabrication because electrical connection to both the top and bottomelectrodes is needed. In addition, because the piezoelectric element issmall and fragile, there is greater risk in the element becoming damagedduring the fabrication process.

Thus, it is desirable to provide a fabrication process for adifferential design that is simple and costs less than conventionalprocesses of fabrication. In addition, it is desirable to provide adifferential design that reduces the risk of damage to the piezoelectricelement.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a methodof fabricating piezoelectric elements each having a wrap-aroundelectrode wherein the piezoelectric elements are formed from a sheet ofpiezoelectric substrate having a top electrode and a bottom electrodecovering an entire top and bottom surfaces,respectively, of thesubstrate. The method includes the steps of:

(a) creating an isolation trench in the top electrode;

(b) exposing the substrate along a first direction to create exposedside surfaces of the substrate wherein the first direction is locatedremotely from the isolation trench;

(c) depositing an electrode on the exposed side surfaces of thesubstrate;

(d) exposing the substrate along a second direction to create secondexposed side surfaces of the substrate wherein the second direction islocate remotely from the isolation trench on the opposite side of theisolation trench from the first direction wherein the first and seconddirections define each element's length; and

(e) exposing the substrate along a third direction at multiple points onthe substrate wherein defines each element's width.

According to a second aspect of the invention, there is provided amethod of fabricating a plurality of piezoelectric elements each havinga wrap-around electrode wherein the process starts with a piezoelectricsubstrate having a top surface covered by a top electrode. The methodincludes the steps of:

(a) creating a discontinuity in the top electrode within a definedlength for each piezoelectric element to be fabricated wherein thediscontinuity divides the top electrode into a first top electrode and asecond top electrode in each defined length;

(b) dicing through the top electrode and the substrate adjacent to eachdiscontinuity to form an exposed side surface;

(c) depositing an electrode on the exposed side surface;

(d) dicing through the top electrode and the substrate substantiallyparallel to the dicing of step (b) but located remotely therefromwherein the dicing steps (b) and (d) define a length of each element;and

(e) dicing through the top electrode and the substrate substantiallyperpendicularly to the dicing of steps (b) and (d) to define the widthof each element.

According to a third aspect of the invention, there is provided a methodof fabricating a plurality of piezoelectric elements each having awrap-around electrode, wherein the process starts with a piezoelectricsubstrate having a top surface covered by a top electrode and a bottomsurface covered by a bottom electrode. The method includes the steps of:

(a) creating a discontinuity in the top electrode within a definedlength for each piezoelectric element to be fabricated;

(b) creating a via through the second top electrode wherein the via iselectrically coupled to the bottom electrode;

(c) dicing through the top electrode, substrate and bottom electrode oneach side of the vias to define each elements length; and

(d) dicing through the top electrode, substrate and bottom electrode ina direction perpendicular to the dicing of step (c) to define eachelements width.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a stacked piezoelectric element.

FIG. 2 is a plan view of a disc drive actuation assembly utilizing apiezoelectric microactuator according to the present invention.

FIG. 3 is a perspective view of a piezoelectric element according to apreferred embodiment of the present invention.

FIG. 4 is a plan view of a sheet of piezoelectric material processedusing the fabrication technique according to a preferred embodiment ofthe present invention.

FIGS. 5-10 illustrate the fabrication process according to a preferredembodiment of the present invention.

FIGS. 11-14 illustrate the fabrication process for multi-layeredmicroactuators according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order for electrical connection to be made only to the top surface ofa piezoelectric element (also known as a die) in a differential design,the electrode on the top surface must be divided into two portions sothat each portion is electrically isolated from the other. In addition,one of the portions must wrap around the substrate of the element and beelectrically coupled to the electrode on the bottom surface of thesubstrate. It is known to fabricate such a wrap around electrode on anindividual element by element basis, i.e., at the die level. Moreparticularly, a top and bottom electrode are disposed on a top andbottom surface, respectively, of a sheet of piezoelectric material orsubstrate. The individual elements or dies are then diced from thesheet. A discontinuity is then formed in the top electrode to create twoportions that are electrically isolated from one another and then thewrap around electrode is formed to electrically couple one of theportions of the top electrode to the bottom electrode. Electricalconnection can then be made to the top and bottom electrodes from thetop surface of the element. The terms “top” and “bottom” are used toindicate relative position and the present invention is not limited toany particular orientation.

FIG. 1 is a simplified perspective view of a stacked piezoelectricelement 10. Element 10 is shown with three axes labeled d31, d32 andd33. A potential difference between spatially separated points along thed33 axis causes expansion or contraction of element 10 in the d31, d32and/or d33 directions, depending on the polarization of thepiezoelectric crystal layers making up element 10. Thus, piezoelectricelement 10 can be used to supply an expanding or contracting force inthe d31, d32 and/or d33 directions based on an applied voltage.

FIG. 2 is a plan view of a disc drive actuation assembly 20incorporating the piezoelectric microactuator according to a preferredembodiment of the present invention. Disc drive actuation assembly 20includes a voice coil motor (VCM) 22, E-block body 25, load beam 34coupled to an actuator arm 30 at head mounting block 32, and gimbal 36coupled to a distal end of load beam 34 to support slider 38, which inturn carries a transducing head. Pivot cartridge 26 is provided incavity 37 in E-block body 25, and is preferably rigidly fastened toE-block body 25 at one end, such as by one or more screws 28.Microactuator 27 is provided on the load beam 34 and includes terminals(not shown) which couple the element to the drive electronics (notshown).

The VCM 22 is operated in a manner known in the art to rotate E-block 25and pivot cartridge 26 around axis 24 and thereby coarsely positionslider 38, and thus transducer head (not shown), over selected tracks 42of a disc 40 rotating around axis 41. For more precise movements,microactuator 27 is selectively expanded or contracted along its d31axis by applying a voltage to the terminals (not shown) coupled to theelement thereby altering the position of the slider 38 and thustransducer head (not shown) with respect to the tracks 42 of disc 40.

FIG. 3 is a perspective view of a piezoelectric element according to apreferred embodiment of the present invention. The element 200 is threedimensional and is rectangular in shape. It has a top surface 202, abottom surface (not shown) and side surfaces coupling the top and bottomsurfaces only two of which 204, 206 are shown. The element 200 is formedby a piezoelectric substrate having a top and bottom electrode formedthereon. The top electrode 202 has a discontinuity 208 formed therein toseparate the top electrode 202 into two portions 202 a, 202 b that areelectrically isolated from one another A wrap-around electrode is alsodeposited or formed on side 206. The remaining sides of which only oneis shown 204, do not have electrodes formed thereon. The wrap-aroundelectrode 206 electrically couples the portion 202 b of the topelectrode 202 to the bottom electrode (not shown). Thus, electricalconnection to the top and bottom electrode are achieved from the topsurface of the element via portions 202 a, 202 b respectively. Aspreviously mentioned, the formation of the wrap-around electrode hasheretofore been created on an element by element basis. Because thefabrication process is at the die level, fabricating elements using thistechnique is costly and time consuming since each element isindividually processed. The present invention, contrarily, createselements with wrap-around electrodes during the creation of theelements, i.e., at the substrate level.

The fabrication process will now be described with reference to FIGS.4-10.

FIG. 4 is a plan view of a sheet of piezoelectric material processedusing the fabrication technique according to a preferred embodiment ofthe present invention. An electrode layer (represented by the non-darkregions) is first formed over the top and bottom surfaces of thepiezoelectric substrate, only the top of which is shown in FIG. 4. Thedark lines 208 represent discontinuities formed in the top electrodewhere the electrode has been removed. The process to create thediscontinuities or trenches in the top electrode is shown in FIGS. 5 and6. First a layer of photoresist 406 is deposited over the top electrode402. The layer of photoresist 406 can be deposited by any well knowntechnique. Then the layer of photoresist 406 is patterned preferablyusing well known photolithography techniques to create openings 407 inthe photoresist layer 406. Then the portion of the top electrode 402located underneath the openings 407 in the photoresist layer 406 isremoved using either a well known wet etch technique or a well known ionmilling technique as shown in FIG. 6. The first layer of photoresist 406is removed and then a new layer 420 of photoresist is deposited orformed over the entire top surface including the discontinuities 407 asshown in FIG. 7. The substrate is mounted to a UV release tape on a tapeframe.

Next the steps for forming or creating the wrap-around electrode will bedescribed according to a preferred embodiment of the invention. Thesubstrate is diced along a first “y dicing” lane 410 to expose sidesurfaces 412 of the substrate. In a preferred embodiment, a tapereddicing blade or saw is used to create the tapered exposed side surfacesshown in FIG. 8. From FIG. 4 it can be seen that the length of eachelement is defined along the y axis and each element's width is definedalong the x axis. The side surfaces are generally tapered relative tothe top surface of the substrate when a tapered dicing blade is used.Alternatively, a regular dicing blade can be used to createperpendicular exposed side surfaces. In FIG. 4, multiple “y dicing”lanes 410 are shown. A wrap-around electrode 422 is then formed onexposed surfaces 412. More particularly, electrode 422 is depositedpreferably by well known sputtering techniques onto side surfaces 412 asshown in FIG. 9. Preferably gold (Au) is sputtered onto the exposedsurfaces at low power and hence low temperature to prevent heating thesubstrate to half of its Curie temperature. The substrate is again dicedat a second “y dicing” lane 414 located remotely from the first “ydicing” lane to define the length of the elements 302 as shown in FIGS.4 and 10. Finally, as seen in FIG. 4, the width of each element isdetermined by “x dicing” lanes 416. The final step involves strippingaway the layer of photoresist 420. Thus a differential piezoelectricelement with top down connection to the bottom electrode is created atthe substrate level as opposed to the element by element technique ofthe prior art.

In a preferred embodiment, with respect to FIG. 3, portion 202 a isabout 0.10 inches and portion 202 b is about 0.005 inches. Thediscontinuity 208 preferably has a length of about 0.005 inches. The “xdicing” lanes 416 are preferably about 0.0015 inches thick and definethe width of each element which preferably is about 0.03 inches. The “ydicing” lanes 410, 414 are also preferably about 0.0015 inches thick anddefine the length of each element which preferably is about 0.110inches.

Alternatively, instead of processing the substrate using dicing andsputtering techniques, ion milling can be used. More particularly, afterthe discontinuities 407 are formed as shown in FIG. 6, a secondphotolithography step results in preferably 0.005 inch diameter circlespatterned in the photoresist on the smaller of the top two electrodes,i.e., portion 202 b (see FIG. 3). The substrate is then ion milled as iswell known through the thickness of the substrate followed by asputtering of gold (Au), or alternatively, a seed layer can be depositedfirst before the gold is sputtered to create vias as are well known. Thevias (not shown) provide the top down connection to the bottomelectrode.

FIGS. 11-14 illustrate the fabrication process for multi-layer actuatorsaccording to a preferred embodiment of the invention. As shown in FIG.11 a substrate 500 having multiple electrodes 502, 504 formed therein isused. Electrodes 502 extend through a major portion of the substrate butdo not extend to the ends of the substrate. Electrodes 504 extend fromeach end of the substrate towards the center but stop short of thecenter of the substrate. A layer of photoresist 506 is deposited on thetop surface of the substrate 500. Alternatively, a shadow mask may beused as is well known to those of ordinary skill in the art. Then, asshown in FIG. 12, the layer of photoresist 506 is patterned to createphotoresist segments 510 which will define isolation trenches as will bedescribed hereinafter. In addition, a “y dicing” lane 520 is cut throughthe substrate 500 to divide the substrate into what will become a firstand second multi-layer microactuator 522, 524 (FIG. 14). Preferably atapered dicing blade or saw is used to create tapered exposed sidesurfaces 530. While the illustrated process is used to create twomicroactuators, the same process is used to create a multitude ofmicroactuators and the present invention is not limited to the shownexamples.

Then, as shown in FIG. 13 a top and side electrode 532, 534,respectively, are deposited preferably using well known sputteringtechniques. Finally, as shown in FIG. 14 the photoresist segments 510are removed and the substrate is further diced to create individualelements. It can be seen in FIG. 14 that the top electrode 532 of eachmicroactuator is divided into two sections 532 a, 532 b which areelectrically isolated from one another. Top electrode 532 a iselectrically coupled to electrodes 504 via wrap-around electrode 504 aand top electrode 532 b is electrically coupled to electrodes 502 viawrap-around electrode 504 b. Electrical connection is made to all of theelectrodes via top electrodes 532 a, b.

There is a danger that gold (Au) coated asperities on the bottom surfaceof the bottom electrode may touch the suspension through the epoxy usedto couple the microactuator to the suspension. Therefore, a lowtemperature dielectric layer is first deposited on the bottom electrodeprior to the above processing steps. Preferably, a dielectric layer ofSiO₂ is deposited using plasma enhanced chemical vapor deposition(PECVD) at 120° C. Alternatively, an epoxy filled with non-conductingparticles may be used to attach the microactuator element to thesuspension, which provides standoffs.

While particular materials and dimensions are provided by way ofexample, the present invention is not limited to those materials anddimensions.

The present invention provides a fabrication process that is on thewafer or substrate level instead of the die level. Thus, a plurality ofdies can be fabricated quickly and cost effectively. In addition, theprobability of damage to the piezoelectric substrate is reduced.

The above specification, examples and data provide a completedescription of the manufacture and use of the invention. Since manyembodiments of the invention can be made without departing from thespirit and scope of the invention, the invention resides in the claimshereinafter appended.

What is claimed is:
 1. A method of fabricating piezoelectric elementseach having a wrap-around electrode wherein the piezoelectric elementsare formed from a sheet of piezoelectric substrate having a topelectrode and a bottom electrode covering an entire top and bottomsurfaces, respectively, of the substrate, the method comprising thesteps of: (a.) creating an isolation trench in the top electrode; (b.)exposing the substrate along a first direction to create first exposedside surfaces of the substrate, wherein the first direction is locatedremotely from the isolation trench; (c.) depositing an electrode on thefirst exposed side surfaces of the substrate; (d.) exposing thesubstrate along a second direction to create second exposed sidesurfaces of the substrate wherein the second direction is locatedremotely from the isolation trench on the opposite side of the isolationtrench from the first direction wherein the first and second directionsdefine a length of each element; and (e.) exposing the substrate along athird direction at multiple points on the substrate and defines a widthof each element.
 2. The method of claim 1 wherein step (a) comprises thesteps of: (a)(i) depositing a first layer of photoresist on the topelectrode; and (a)(ii) etching away the first layer of photoresist tocreate the isolation trench.
 3. The method of claim 1 further comprisinga step (f) of depositing a second layer of photresist on the optelectrode and isolation trench before step (b).
 4. The method of claim 2wherein step (a)(ii) is a wet etch.
 5. The method of claim 1 whereinstep (b) comprises the step of dicing through the top electrode,substrate and bottom electrode.
 6. The method of claim 4 wherein step(c) comprises the step of sputtering the electrode on the exposed sidesurface of the substrate.
 7. The method of claim 1 wherein step (b)comprises the step of ion milling through the top electrode and thesubstrate along the first direction to create vias.
 8. The method ofclaim 7 wherein step (c) comprises the steps of (c)(i) first depositinga seed layer in the vias followed by a step (c)(ii) of sputtering theelectrode on the seed layer.
 9. The method of claim 1 wherein steps (d)and (e) comprise the step of dicing through the top electrode, substrateand bottom electrode.
 10. A method of fabricating a plurality ofpiezoelectric elements each having a wrap-around electrode wherein theprocess starts with a piezoelectric substrate having a top surfacecovered by a top electrode, the method comprising the steps of: (a.)creating a discontinuity in the top electrode within a defined lengthfor each piezoelectric element to be fabricated, wherein thediscontinuity divides the top electrode into a first top electrode and asecond top electrode in each defined length; (b.) dicing through the topelectrode and the substrate adjacent to each discontinuity to formexposed side surfaces of the top electrode and the substrate; (c.)depositing an electrode on the exposed side surfaces; (d.) dicingthrough the top electrode and the substrate substantially parallel tothe dicing of step (b) but located remotely therefrom, wherein thedicing steps (b) and (d) define a length of each element and the dicingstep (d) occurs after the depositing step (c); and (e.) dicing throughthe top electrode and the substrate substantially perpendicularly to thedicing of steps (b) and (d) to define the width of each element.
 11. Themethod of claim 10 wherein the step (a) comprises steps of: (a)(i)depositing a first layer of photoresist on the top electrode; and(a)(ii) etching away the first layer of photoresist to creatediscontinuity.
 12. The method of claim 10 further comprising a step (f)of depositing a second layer of photoresist on the top electrode anddiscontinuity before step (b).
 13. The method of claim 11 wherein step(a)(ii) is a wet etch.
 14. The method of claim 13 wherein the step (c)comprises the step of sputtering the electrode on the exposed sidesurface of the substrate.
 15. A method of fabricating a plurality ofpiezoelectric elements each having a wrap-around electrode, wherein theprocess starts with a piezoelectric substrate having a top surfacecovered by a top electrode and a bottom surface covered by a bottomelectrode, the method comprising the steps of: (a.) creating adiscontinuity in the top electrode within a defined length for eachpiezoelectric element to be fabricated; (b) dicing through the topelectrode, substrate and bottom electrode on each side of thediscontinuity to define a length of each element and to expose sidesurfaces of the top electrode, substrate and bottom electrode; (c)forming an electrode layer on the exposed side surfaces to electricallyconnect the top and bottom electrodes; and (d) dicing through the topelectrode, substrate and bottom electrode in a direction perpendicularto the dicing of step (c) to define a width of each element.
 16. Themethod of claim 15 wherein step (a) comprises: (a)(i) depositing a firstlayer of photoresist on the top electrode; and (a)(ii) etching away thefirst layer of photoresist to create discontinuity.
 17. The method ofclaim 15 further comprising a step (e) of depositing a second layer ofphotoresist on the top electrode and discontinuity before step (b). 18.The method of claim 16 wherein step (a)(ii) is a wet etch.